Traditional optical components such as glass lenses shape the wavefront of an incident light beam using variations in thickness to create different optical path lengths. The thickness of such components is much larger than the wavelength. These traditional optical components are bulky and suffer from various well-known effects such as chromatic aberration. Advances in technology in recent decades has led to a new class of flat, ultrathin optical components based on metasurfaces. A metasurface is a quasi-two-dimensional structure composed of resonant optical antenna elements arranged to form subwavelength-patterned structures that interact strongly with light. Metasurfaces can be used to manipulate the propagation of light by spatially varying geometric parameters of the structures (e.g., antenna shape, size, orientation) to provide a spatially varying optical response that molds optical wavefronts as desired. The optical antenna elements alter light propagation by inducing localized phase discontinuities (i.e., abrupt changes of phase over a distance comparable to the wavelength of the light). These optical resonator antennas may be composed of different types of materials and may operate based upon different physical principles.
One type of metasurface optical component is space-variant (or gradient) metasurfaces capable of beam steering and focusing (i.e., manipulating an incident light beam to generate a desired optical intensity distribution in the far-field). Such optical components have been constructed from nanoscale metallic antennas. These designs, however, suffer from Ohmic losses in the metal. Typically silver and gold are selected as the plasmonic materials because they have large free-electron concentrations and high electrical conductivities. These metals, however, only work well in the infrared and microwave spectral regions. At and near visible wavelengths, they suffer from high losses arising in part from interband transitions. Also, due to the limited scattering cross sections of the antennas, these devices have efficiencies only in reflection mode. Consequently, they are unsuitable for use in transmission mode. In addition, noble metals are not compatible with the traditional semiconductor processing technologies.
Another type of metasurface is all-dielectric Huygens metasurfaces based on a single layer of dielectric silicon disks. These can provide efficient wave-front manipulation and laser pulse compression. These metasurfaces use optical resonances to impart a phase to an incident light wave. For every resonant structure, the phase delay varies spectrally across the optical resonance as well does the transmission amplitude. The resulting devices, however, can operate only over a narrow frequency range. Although the structures are transmissive, they are efficient only in the infrared spectral range, where the Si material is less absorptive. Because the frequency of the useful resonances are more sensitive to the geometry size at shorter/visible wavelengths, it becomes increasingly difficult to realize such devices in the visible range. In addition, the relatively large size of the unit cell design makes it difficult to design a optical element with high numerical aperture.
In earlier work by one of the present inventors, a Pancharatnam-Berry (PB) phase optical component was fabricated using a computer-generated space-variant subwavelength dielectric grating. Wavefront shaping based on Pancharatnam-Berry phase is fundamentally different from conventional optical-path-length approaches of standard lenses as well as other types of metasurfaces. A Pancharatnam-Berry phase optical component device does not rely on resonant effects to induce the phase shift. Instead, the phase shift results from a so-called geometric of Pancharatnam Berry phase. Such a geometric phase is achieved by having a spatial gradient in the orientation of the constituent nanostructures. The Pancharatnam-Berry phase is a phase shift that results from a manipulation of the polarization state. One characteristic feature is that the Pancharatnam-Berry phase is determined only by the geometry of the polarization path. Pancharatnam-Berry phase optical elements (PBOE) for wavefront shaping, however, have been implemented only in the mid-infrared domain, using subwavelengths inhomogeneous gratings to manipulate the polarization. In addition, these devices have only been implemented thus far in somewhat thick layers, i.e., significantly more than 100 nm.
In view of the above, there is a need in metasurface technology for new ultrathin metasurface designs and principles that provide for high efficiency visible wavelength transmission wavefront shaping with material systems that are compatible with semiconductor fabrication techniques.